Field of the Invention
This invention relates generally to a device for fixing a turbine blade's position relative to a rotor disk in a combustion gas turbine and, more particularly, to an adjustable blade root spring device which can be freely inserted in a space beneath the blade root, then compressed via an axial bolt so that an accordion-shaped spring increases in height and presses the blade radially outward relative to the rotor disk, thus positively engaging the blade root with its mating surfaces in the disk even when no centrifugal load is present.
Description of the Related Art
Combustion gas turbines are clean-burning, efficient devices for generating power for a variety of applications. One common application of combustion gas turbines is in power plants, where the turbine drives a generator which produces electricity. Such stationary gas turbines have been developed over the years to improve reliability and efficiency, but the continuous improvement quest never ends.
Turbine blades are airfoils which are arranged circumferentially around a rotor disk inside the turbine, where rows of rotating blades are alternately positioned between rows of stationary turbine vanes. Because turbine blades are directly exposed to combustion gases, they get extremely hot. Blades are also subject to combustion gas pressure, centrifugal force and vibration. Thus, turbine blades may become damaged or worn over time, and they therefore need to be easily replaceable.
A common and reliable design for the attachment of turbine blades to the rotor disk is where the blade root has an inverted “fir tree” shape, and the disk has a complementary fir tree shaped cavity. With this design, a blade can be installed in a disk by simply sliding the blade in a longitudinal direction (parallel to the rotational axis of the turbine) so that the blade root fir tree engages with the mating cavity in the rotor disk. In this design, there is necessarily some looseness between the blade root and the disk cavity, both to allow for easy installation and removal, and to allow for differing radial growths due to thermal expansion and/or centrifugal forces. When the turbine is running at operational speed, centrifugal force pulls the blades radially outward so that the looseness is all taken up, and contact points on the branches of the fir tree are pressed tightly against each other. However, turbines are sometimes operated in a low-speed “stand by” mode, where the centrifugal force of rotation is not enough to overcome the force of gravity, and as a result, each blade experiences radial inward/outward and rocking movements on each rotation of the turbine. Over long durations, these repeated movements of the blade relative to the disk cause excessive wear on contact points of the blade fir trees and disk cavities, as well as on blade tip shrouds.